Prillilgirl, by Carolyn Wells (epub/Kindle/pdf)

A popular playwright is found stabbed with his own pen, made from a medieval dagger. Unconscious on the floor of a telephone booth lies Mrs. Guy Thorndike, wife of a prominent actor. On the handle of the weapon, the door of the booth, and the cover of the telephone book, are prints of small bloody fingers which experts identify as hers. Why should Prilligirl kill Mallory Vane when he had just completed the play which is to crown her husband’s triumphs? And why does Thorndike confess to the murder? In the room, unheeded by police and sleuths, is a clue upon which Fleming Stone constructs a daring solution.